Save the amoeba?

Should microbial biodiversity (i.e., microscopic organisms like bacteria, archea, protists, and some fungi) be more directly considered in conservation efforts? Certainly the importance of microscopic organisms in the earth system cannot be overstated, as they play vital roles in global element cycles – fixing atmospheric nitrogen and making it available to plants, returning nitrogen back to the atmosphere, playing important roles in food webs, and facilitating the decomposition of organic matter. Furthermore, microorganisms are likely to become a critical source for new pharmaceuticals in the future (Chivian & Bernstein, 2008). However, only rarely have microorganisms been included in biodiversity surveys, and although the International Union for Conservation of Nature (IUCN) recently added four critically endangered protists to the Red List (here, here, here, and here), a public outcry for their protection appears unlikely any time soon.

Of course the lack of emphasis on microorganisms in conservation biology is not particularly surprising, given that 1) we know relatively little about microbial diversity, 2) microbes are perhaps slightly less cute and charismatic than many other organism groups (although they are certainly cuter than some species), and 3) it has been widely assumed that microbial species have cosmopolitan distributions. In other words, they are found everywhere. More specifically, a microbial species occurs anywhere that suitable habitat exists for that particular species. In 1934, Baas Becking simply stated this hypothesis as: “everything is everywhere, but the environment selects” (de Wit and Bouvier 2006). What is implied by this hypothesis is that dispersal does not limit the distribution of microbes, and endemism (i.e., being confined to a particular region) should be rare. Of course, if everything is everywhere, then we don’t need to worry much about any particular microbial species, because extinction is extremely unlikely unless the destruction of all suitable habitats occurs…across the entire planet. Let’s hope that remains unlikely for some time.

However, research now clearly indicates that microbial biogeography is more nuanced and complex than implied by the “everything is everywhere” hypothesis. Many examples of limited geographic ranges have emerged over the past couple decades. Some particularly striking examples are within the testate amoebae, a group of shell-producing protists that have been relatively well studied, at least in comparison to many other microbial groups. For example, Apodera vas and species of the genus Certesella are restricted to the southern hemisphere and tropics, although suitable habitats for these species exist in Eurasia and North America (Smith and Wilkinson 2007). The patterns suggest that these species had their origins on the continent of Gondwana sometime after it separated from Laurasia about 200-180 million years ago, and thus their current distribution may be a reflection of this geological and evolutionary history (Smith and Wilkinson 2007). Apparently these species have been unable to disperse to North America or Eurasia.

A paleogeographic reconstruction of the Earth in the late Triassic, shortly before the separation of Gondwana (southern continents) from Laurasia (northern continents). The present distribution of Apodera vas and species of the genus Certesella is nearly restricted to the land that was once part of Gondwana, suggesting that they evolved on this landmass (Smith & Wilkinson 2007). (Image from wikipedia.org).

A potentially more restricted distribution pattern is that of Nebela ansata, an unusual (and dare I say, “charismatic”) species of testate amoebae known only from a few locations in eastern North America. In 1874, Joseph Leidy first encountered this species from collections he made in the New Jersey Pine Barrens (Leidy 1879). Since his initial descriptions, the species has only been recorded in the literature three times, always in temperate eastern North America, and always living in wetlands on moist Sphagnum moss. The species has gone unrecorded in the literature since the early 1950s, despite the intensive sampling of wetland testate amoeba communities (Heger et al. 2011).

The search for a forgotten microbe

Nebela ansata from Webb’s Mill Bog in the New Jersey Pine Barrens. The horn-like lateral extensions are characteristic for the species. (Photo: Thierry Heger)

The New Jersey Pine Barrens are only a few hours from Lehigh University, so a few years ago we set out to see if we could find Leidy’s elusive and long-forgotten microbe. His original samples were from cedar swamps near the town of Absecom (now spelled Absecon), and we used that as a guide when selecting potential wetlands for our sampling effort. He also mentioned that Nebela ansata was commonly found in association with Nebela carinata, a more widely distributed species. The ecology of Nebela carinata has been well characterized, and it prefers very wet Sphagnum moss and is often abundant where the water table is only a few centimeters below the moss surface. We used this knowledge to target likely habitats within each of the wetlands that we sampled. Maura Sullivan (Lehigh University, PhD student) and I sampled for two full days, returned the samples to the lab for examination, and managed to find the species!

Interesting, at about the same time as our rediscovery of Nebela ansata in the New Jersey Pine Barrens, the species was also discovered in Nova Scotia by Barry Warner and Taro Asada (University of Waterloo). A collaborative effort soon took shape, with the goal of pulling together all the known information on the distribution, ecology, and phylogeny of the species. Led by Thierry Heger, then a PhD student (Swiss Federal Research Institute WSL & The University of Geneva), an exhaustive literature and museum survey was undertaken and used in tandem with analyses of the samples from Nova Scotia and Pine Barrens to provide insight into the biogeography and ecology of Nebela ansata. Together, the the results provide a compelling case of a microorganism with a very limited distribution range – apparently limited to temperate, eastern North America – representing a very clear exception to the ‘everything is everywhere’ hypothesis (Heger et al. 2011).

Webb’s Mill wetland in the New Jersey Pine Barrens, where individuals of the rare testate amoeba species, Nebela ansata, were found.

How many other microorganisms also have geographically restricted distributions? What are the primary causes of these restricted distributions? How many endemic species remain undescribed? What role do these relatively rare microorganisms play in ecosystems? How many microbial species are vulnerable to habitat destruction or other global changes? Much work remains to be done in biodiversity research.